Silicic Acid, and Zirconic Acid. S68 



(crystallized titanic acid), which is also isomorphous with tinstone. 

 According to the concordant measurements of Miller and Kok- 

 scharow, the angles of the terminal edge of the fundamental form 

 are 123° 8', and hence only differ by 11 minutes from those of 

 the angles of zircon. It is true that hitherto the characteristic 

 twin crystals of tinstone have not been met with in zircon ; and 

 in this respect a difference between these forms might be found. 

 But it must be remarked that the twin crystals of tinstone only 

 occur in certain places^ as in the ErzgebirgCj while they have 

 not been found in the tinstone of other districts, as for example 

 Cornwall. The same may also be the case with zircon ; and the 

 places in which it occurs in twin crystals may hitherto have 

 remained undiscovered. 



If there is such an agreement in crystalline form and structure 

 in tinstone and zircon, it is very probable that the chemical com- 

 position in both is analogous. Both have, it is true, hitherto 

 been regarded as compounds of entirely different nature — tin- 

 stone as stannic acid, zircon as silicate of zirconia; and zirconia 

 has even been considered as a sesquioxide. Notwithstanding 

 this, Deville* has found it necessary to assume two double atoms 

 of chlorine in chloride of zirconium, since it is only by this 

 assumption that the condensation of its vapour agrees with that 

 of similarly constituted vapours, and with that of chloride of 

 silicon. But if chloride of zirconium is Zr CI*, zirconia must be 

 ZrO^, aud can no longer be called zirconia, but zirconic acid. 

 Zircon, the formula of which has hitherto been Zr* 0^ SiO^, 

 becomes now an isomorphous compound of one atom of zirconic 

 acid and one atom of silicic acid, and is therefore Zr 0* + SiO*; 

 just as chrysoberyll is a similar isomorphous compound of one 

 atom glucina with three atoms of alumina; for most of the 

 analyses have always hitherto shown that zircon has such a com- 

 position that the oxygen of the zirconic acid is equal to that of 

 the silicic acid. But this relation does not prevail everywhere. 

 Hermann has investigatedf the zircon which occurs in the vil- 

 lage of Anatolia in the government of Jekatherinoslaw, aud has 

 found that it consists of two atoms zirconic acid and three atoms 

 silicic acid, and is thus 2ZrO* + 3SiO^ which led him to desig- 

 nate it as a special mineral, and name it Auerbachite. It only 

 occurs crystallized in the fundamental form, which, according to 

 Aucrbach, has angles of 121° on the terminal edges, and in this 

 approximates very closely to that of tinstone. It is found in 

 individual crystals in the quartzose schist. Its specific gravity, 

 according to Hermann, is 4.*06 ; while that of ordinary zircon, 



* Comptes liendus, vol. xlv. p. 821. 



t Compare Erman, Arohiv fur wissensoha/tliche Kunde von Russland 

 vol. xvii. p. 568. 



